Reversible Operation of Solid Oxide Cells for Sustainable Fuel Production and Solar/Wind Load-Balancing

Abstract

The solid oxide electrochemical cell (SOC) is a promising candidate for large-scale energystorage. In electrolysis mode it stores renewable electricity as chemical energy in the formof fuels like hydrogen and hydrocarbons, and the same cell can be operated in the reversedirection to produce electricity from fuels – either previously stored fuels or from anexternal supply e.g. natural gas or biogas. This reversibility combined with fuel-flexibility isunique among energy storage technologies like closed-system batteries and singledirectionelectrolyzers. However, few studies have been conducted with focus onfundamentals or applications of bi-directional operation. This presentation will highlight ourrecent developments in applying reversible SOCs (RSOCs) for renewable energy storagewith respect to cell and stack testing, cell and system design, and techno-economicanalysis.At the cell level, long-term testing has shown that improved stability can be achieved byreversible operation compared with steady-state electrolysis operation. Further, we havedeveloped novel Ni-free fuel-electrodes that both outperform conventional Ni-basedelectrodes and do not catalyze carbon deposition, which opens the door to advancedapplications of RSOCs that utilize carbonaceous fuels.At the stack level, we have demonstrated operation that follows real-world time-serieselectricity supply and demand data, considering a 100% renewable energy scenario wherewind power is the only power supply. When the wind power supply exceeds demand, theRSOC stack produces syngas via co-electrolysis of CO2 and H2O. Part of the syngas isconverted to methanol downstream in the system to meet the demand of transportationvehicles, and the rest is stored for electrical load balancing by conversion back toelectricity in fuel cell mode when electricity demand exceeds the wind power supply.At the system level, techno-economic analyses and system designs for different scalesand applications have been realized. A simulation of an RSOC system that uses real-worldtime-series market prices for electricity and natural gas in Denmark to decide when tooperate in electrolysis mode (buying electricity and selling methane) or fuel-cell mode(buying gas and selling electricity) shows the advantage of a reversible system and thechanging operating profile as the fraction of wind power supply grows. Finally, we discussthe potential for systems with novel chemistries and components to compete with stateof-the-art rechargeable batteries with respect to cost and round-trip efficiency.